This section provides background information related to the present disclosure and is not necessarily prior art.
Many recreational and marine work force boats utilize stern drive propulsion systems to generate locomotion. Specifically, many boats use cone-clutch drives, which implement an opposing pair of straight-cut bevel gears to translate forward rotation of an input shaft from an engine into both forward and reverse rotation of an output shaft, which may be connected to a propeller.
In operation, the speed of the engine and the drive must be modulated in different surface conditions, as the water produces resistance to the drive through the propeller(s). Calm surface conditions create somewhat stable resistance forces. However, choppy or larger wave conditions create intermittent resistance forces, especially as boat speed increases.
Proper operation of a boat involves utilizing the throttle to increase power and speed while the drive is submerged, and reducing the throttle input if the drive is out of the water. For example, as a speed of the boat increases and the boat achieves plane, the propeller(s) can leave the water, which significantly reduces the resistance to the drive and causes the engine to “over-rev.” The operator must then reduce throttle input and engine/propeller speed to more closely match the actual boat speed prior to the propeller(s) returning and reengaging with the water. In instances where the speed of the propeller(s) does not match the actual speed of the boat prior to reengaging the water, the load applied to the propeller creates a form of shock, which is ultimately transmitted from the propeller through the various components of the drive. This is a regular occurrence, and the modulation technique is preferably implemented regardless of engine horsepower, torque or drive specification limits.
In addition to stresses imposed by intermittent engagement of the propeller with the water, stresses imposed on marine drive components may be magnified by increased engine performance. Marine stern drive units provided by Original Equipment Manufacturers (OEM) are generally designed according to the horsepower and torque outputs of an associated OEM engine. However, as boat engines are tuned and modified by third parties to maximize performance, the stresses imposed on the components of the drive may exceed the original design parameters. Accordingly, the aforementioned stresses introduced by intermittent engagement of the propeller with the water are compounded by increased power output of the engine.
Although many components of marine drives have been provided with design improvements, cone-clutch gear sets have yet to be addressed, and are a common weak link of marine drives. Generally, the weakness in cone-clutch drives is in one specific area, the roots at the toe of the teeth in the straight-cut bevel gear. Given the taper of teeth in straight-cut bevel gears, the toe of the gear tooth has a smaller cross-sectional area and exterior surface area, which, in part, leads to higher stress concentrations relative to the other portions of the tooth. Higher stress at the toe of the tooth is also due in part from higher loads applied to the toe of the tooth due to the distribution of a constant torque.
Manufacturability is a constraint on the development of cone-clutch gear sets. OEM marine drive gear and pinion sets are typically manufactured using near net forging. Although near net forging is a cost effective way of gear production and results in relatively high-strength grain structures, it is not without drawbacks. For example, near net forging results in relatively rough surface finishes with poor tolerances, which in turn reduces rolling contact surfaces and increases friction between meshing gear faces. Further, near net forging may result in surface blemishes, which act as stress risers when the teeth are subjected to a load. The additional tangential force and stress risers can cause micro-fractures along the surface of the tooth which leads to pitting fatigue on the contact surface.
Due to the manufacturing constraints of near net forging processes, the toes of the bevel gear teeth are independently formed from each other, and must individually accommodate imposed stresses. As a result, the unsupported teeth generally exhibit evidence of fatigue at the root, as the toes of each of the individual teeth are repeatedly subjected to stresses beyond their design specifications.